Increasing concern with soil and groundwater contamination and governmental mandated requirements to clean up hazardous waste sites have made clear that rapid and cost effective methods for subsurface characterization of subsurface chemical contaminants are needed. Traditional methods for monitoring hazardous wastes in ground water systems have involved the collection of field samples and subsequent analysis in the laboratory, see for example, the texts by R.A. Freeze and J.A. Cherry, Groundwater, 604 pp., Prentice-Hall, Inc., Englewood Cliffs, N.J., 1979 and M.J. Barcelona, J.P. Gibb, J.A. Helfrich and E.E. Garske, Practical Guide for Ground-water Sampling, EPA Rep. EPA/600/S2-85/104, 94 pp., EPA, Washington, D.C., 1985. However, much current research has focused on the development of real-time, remote, in situ techniques to monitor both organic and inorganic pollutants.
Traditionally, samples are often collected "blind" without a priori knowledge about the exact location and extent of contaminant plumes. Zones or plumes of contamination can be completely missed, or, if pinpointed, overestimated or underestimated. For more detailed spatial information on contaminated areas, those areas must often be sampled and analyzed in an iterative manner. Such an approach can be prohibitively costly and labor-intensive. In a highly dynamic aquatic system, the delay from sampling to interpretation and remediation can severely hamper response time, or hinder contaminant containment, possibly resulting in a much larger extent and expense of cleanup.
Real-time in situ measurements, on the other hand, allow for rapid interpretation of the distribution of contaminants, through close sampling and thorough mapping of contaminated areas. The transport of contaminants in soils and/or ground water can be monitored and predicted, and cleanup operations can be efficiently planned and directed. Furthermore, remote in situ methods minimize the risk of sampling artifacts and allow for analysis in hostile environments, without exposing personnel to toxic contaminants.
One method for measuring contaminants which has been extensively discussed is the use of fiber-optic guided systems for in situ spectroscopy and chemical sensing, see the articles by T. Hirschfeld, T. Deaton, F. Milanovich and S. Klainer, entitled "The Feasibility of Using Fiber Optics for Monitoring Groundwater Contaminants," Optical Eng., vol. 22, pp. 527-531, (1981); W. R. Seitz, entitled "Chemical Sensors Based on Fiber Optics," Anal. Chem., vol. 56, pp. 16A-34A (1984); O. S. Wolfbeis, entitled "Fluorescence Optical Sensors in Analytical Chemistry," Trends in Analytical Chemistry, vol. 4, pp. 184-188 (1985); and S. H. Lieberman, S. M. Inman and E. J. Stromvall, entitled "Fiber Optic-Fluorescence Sensors for Remote Detection of Chemical Species, in Seawater," in Proceedings of the Symposium on Chemical Sensors, vol. 87, Electrochemical Society, Pennington, N.J., pp. 464-473 (1987); and F. P. Milanovich, P. F. Daley, K. Langry, B. W. Colston Jr., S. B. Brown and S. M. Angel, "A Fiber Optic Sensor for the Continuous Monitoring of Chlorinated Hydrocarbons," in Field Screening Methods for Hazardous Wastes and Toxic Chemicals, Second International Symposium, pages 43-48 (1991).
One new technology for rapid, in situ subsurface screening of hazardous waste sites is the use of a cone penetrometer system equipped with an optical chemical sensor system. Conventional cone penetrometers have been used for many years to make measurements of soil strength characteristics, see the articles by R. S. Olsen and J. V. Farr, "Site Characterization Using the Cone Penetrometer Test," in Proceedings of the ASCE Conference on Use of In-situ Testing in Geotechnical Engineering, American Society of Civil Engineers, New York (1986); and P. K. Robertson and R. G. Campanella, Guidelines for Geotechnical Design Using the Cone Penetrometer Test and CPT with Pore Pressure Measurement, Fourth Ed., 185 pp. Hogentogler & Co., Inc., Columbia, Md. (1989).
Recently, a cooperative effort with many participants has developed a prototype cone penetrometer system that has been modified to accommodate a laser induced optical fiber fluorometer for real-time, in situ measurement of chemical contaminants in soils. The prototype system has been used to demonstrate the feasibility of real-time in situ fluorescence measurements of POL (petroleum, oils, and lubricants) in soils as the probe is pushed into the ground to depths of up to 150 feet. The fiber optic system used in the prototype penetrometer has been described in the article by S.H. Lieberman, G.A. Theriault, S.S. Cooper, P.G. Mallone, R.S. Olsen and P.W. Lurk. entitled "Rapid, Subsurface in situ Screening of Petroleum Hydrocarbon Contamination Using Laser Induced Fluorescence over Optical Fibers," in Field Screening Methods for Hazardous Wastes and Toxic Chemicals, Second International Symposium, 57-63, 1991.
In brief, the prototype cone penetrometer system uses two silica clad silica UV/visible-transmitting optical fibers. One fiber is used to carry excitation radiation down through the center of the penetrometer probe and a second fiber collects the fluorescence generated in the soil sample and carries it back to the detector system. The two fibers are separated from the soil sample by a sapphire window mounted flush with the outside of the probe. In the prototype system excitation radiation is provided by a pulsed N.sub.2 laser. A photodiode array detector coupled to a spectrograph is used to quantify the resulting fluorescence emission spectrum. While other fiber-optic guided chemical sensors are being developed for use in groundwater contaminant studies, this is the first reported direct optical detector for contaminants in soils. Unlike sensing systems which examine the contaminant levels in monitoring wells, such as referred to in W. A. Chudyk's, "Field Screening of Hazardous Waste Sites," Env. Sci. and Tech., vol. 23 pages 504-505 (1989), the Lieberman et al. system allows for measurements in soils before monitoring wells are drilled, and, as a consequence is independent of the fractionation and transport problems inherent in using well samples to determine contaminant levels on and between soil particles. However, the prototype core penetrometer system does not take into account the influence of soil type and condition on the readings obtained.
Thus, a continuing need exists in the state of the art for a method providing an improved quantification of the measured optical response of chemical sensors for in situ measurement of chemical constituents in soils which is dependent on critical parameters of soil type and conditions.